The present invention relates to an apparatus and a method for calculating an initial correction coefficient as well as to a program for calculating an initial correction coefficient. More particularly, it relates to an apparatus and a method for calculating an initial correction coefficient as well as to a program for calculating an initial correction coefficient which are used in an apparatus for detecting decrease in tire air-pressure (DWS) for detecting decompression of a tire on the basis of rotational (wheel speed) information of four wheel tires mounted to a vehicle or in an apparatus for calculating a slip rate which calculates a slip rate of driving wheel tires from among four wheel tires mounted to a vehicle.
An apparatus for detecting decrease in tire air-pressure (DWS) conventionally employs a theory that a rotational velocity or a rotational angular velocity of a decompressed tire is increased when compared to remaining normal tires owing to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire of normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires,DEL={(F1+F4)/2−(F2+F3)/2}/ {(F1+F2+F3+F4)/4}×100(%)  (1)is employed as a judged value DEL (reference should be made to Japanese Unexamined Patent Publication No. 305011/1988). Here, F1 to F4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
Since tires are manufactured to include variations (initial differences) within standards, effective rolling radii of the respective tires (a value obtained by dividing a distance which has been traveled by a single rotation by 2π) are not necessarily identical to one another even though all of the tires are at normal internal pressure. This will result in variations in the rotational angular velocities Fi of the respective tires. There is known a method for eliminating influences of initial differences from rotational angular velocities Fi (reference should be made to Japanese Unexamined Patent Publication No. 249010/1997). In this method, the following initial correction coefficients K1, K2, K3 are first calculated.K1=F1/F2  (2)K2=F3/F4  (3)K3=(F1+K1×F2)/(F2+K2×F4)  (4)
By using the thus calculated initial correction coefficients K1, K2 and K3, new rotational angular velocities F1i are obtained as represented by equations (5) to (8).F11=F1  (5)F12=K1×F2  (6)F13=K3×F3  (7)F14=K2×K3×F4  (8)
Here, initial correction coefficient K1 is a coefficient for correcting differences in effect rolling radii owing to initial differences between right and left front tires. Initial correction coefficient K2 is a coefficient for correcting differences in effect rolling radii owing to initial differences between right and left rear tires. Initial correction coefficient K3 is a coefficient for correcting differences in effect rolling radii owing to initial differences between the front left tire and the right and left rear tires.
While an initial correction coefficient which is calculated on the basis of data obtained when the vehicle is performing straight-ahead running and an initial correction coefficient which is calculated on the basis of data when performing turning movements alone are different from each other, the initial correction coefficients when performing straight-ahead running and those when performing turning movements are increased and decreased by the same ratio. The initial correction coefficients have thus been obtained in the prior art by utilizing the fact that judged values when performing straight-ahead running and those when turning movements become the same values as a result of calculation by using the above equation (1).
However, variations in rotational angular velocities are caused due to influences of differences in load shift applied onto the tires during turning movements at high velocity and differences in slip rates between inner and outer wheels of the driving wheels when performing turning movements at high velocity. When calculating initial correction coefficients upon incorporating such data during turning movements at high velocity, it will accordingly be impossible to obtain accurate initial correction coefficients.